Underworld region (6)
I believe the answer is:
erebus
'underworld region' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erebus that I've seen before include "Level of the underworld" , "Sure, be in the underworld" , "God of darkness - cavern on the way to Hades - active volcano in Antarctica" , "In place of darkness" , "Volcanic mountain in Antarctica" .)